This invention relates to perfluoroelastomer articles having improved surface lubricity. More specifically, this invention relates to perfluoroelastomer articles coated with a thin film of a composition comprising a polysiloxane and a urethane oligomer.
Perfluoroelastomers have achieved outstanding commercial success and are used in a wide variety of applications in which severe environments are encountered, in particular those end uses where exposure to high temperatures and aggressive chemicals occurs. For example, these polymers are often used in seals for aircraft engines, in oil-well drilling devices, and in sealing elements for industrial equipment used at high temperatures.
The outstanding properties of perfluoroelastomers are largely attributable to the stability and inertness of the copolymerized perfluorinated monomer units that make up the major portion of the polymer backbones in these compositions. Such monomers include tetrafluoroethylene and perfluorinated vinyl ethers. In order to develop elastomeric properties fully, perfluoroelastomers are typically crosslinked, i.e. vulcanized. To this end, a small amount of cure site monomer is copolymerized with the perfluorinated monomer units. Cure site monomers containing at least one nitrile group, for example perfluoro-8-cyano-5-methyl-3,6-dioxa-1-octene, are especially preferred. Such compositions are described in U.S. Pat. Nos. 4,281,092; 4,394,489; 5,789,489; and 5,789,509.
In certain end use applications, a cured perfluoroelastomer article may undesirably adhere to the surface of other materials that are in contact with it, particularly if the perfluoroelastomer article and other material are forcibly held in contact for a long period of time at elevated pressures and temperatures, and then cooled. It may be difficult, or impossible, to easily remove the perfluoroelastomer article from contact with the other material without damaging the perfluoroelastomer article, other material, or both. Also, the perfluoroelastomer article may be undesirably pulled from the groove in which it is seated when the article is being separated from a surface to which it has adhered. Thus, there is a need for perfluoroelastomer articles that have an improved, less sticky surface.
Kishino et al. (U.S. Pat. No. 5,763,068) disclose fluororesin-coated fluoroelastomer articles. In contrast to perfluoroelastomers, fluoroelastomers contain copolymerized units of at least one monomer which is either partially fluorinated (such as vinylidene fluoride), or non-fluorinated (such as propylene). The articles are made by first blending uncured fluoroelastomer with up to 50 parts fluororesin; shaping the blend into an article and curing it; coating the cured article with an aqueous fluororesin dispersion; drying the coated article; and finally heating the article above the melting point of the fluororesin in order to form a durable fluororesin film coating. However, blending a high level of fluororesin with a fluoroelastomer or perfluoroelastomer typically reduces the physical properties of cured articles produced from such blends, especially if the articles are used in a high temperature environment. Compression set resistance and elongation are particularly adversely effected. Heating fluoroelastomer articles to a temperature of about 300xc2x0 C. or higher, in order to melt the fluororesin, may further degrade the physical properties of the articles.
Tarney et al. (WO 02/20650) disclose fluoropolymer coated perfluoroelastomer articles which have good physical properties and a low stick surface. However, in demanding applications which require excellent high vacuum sealing performance, Tarney""s articles may leak due to the nature of the surface coating. Thus, there is a need for perfluoroelastomer articles that have an improved, less sticky surface and which have excellent sealing performance in high vacuum applications such as in some semiconductor manufacturing equipment.
An aspect of this invention is a cured perfluoroelastomer article having a surface coated with a film comprising a polysiloxane and a urethane oligomer.
Another aspect of this invention is a process comprising:
A. coating a cured perfluoroelastomer article with a dispersion comprising i) a polysiloxane having pendant hydroxyl groups and ii) a blocked isocyanate to form a wet-coated perfluoroelastomer article;
B. drying said wet-coated perfluoroelastomer article to form a dry-coated perfluoroelastomer article at a temperature below which said pendant hydroxyl groups react with said blocked isocyanate to form urethane oligomer; and
C. heating said dry-coated perfluoroelastomer article to a temperature and for a sufficient period of time where said pendant hydroxyl groups react with said blocked isocyanate to form urethane oligomer and a durable film comprising said polysiloxane and urethane oligomer on the surface of said perfluoroelastomer article.